Concrete is the most consumed man-made material in the world. A typical concrete is made by mixing Portland cement, water and aggregates such as sand and crushed stone. Portland cement is a synthetic material made by burning a mixture of ground limestone and clay, or materials of similar composition in a rotary kiln at a sintering temperature of around 1,450° C. Portland cement manufacturing is not only an energy-intensive process, but also one that releases considerable quantities of greenhouse gas (CO2). The cement industry accounts for approximately 5% of global anthropogenic CO2 emissions. More than 60% of such CO2 comes from the chemical decomposition or calcination of limestone.
Recently, a revolutionary form of cement that is based on carbonatable calcium silicate materials has emerged as a promising substitute to traditional cements. Production of carbonatable calcium silicate-based cements involves significantly reduced CO2 emissions and energy consumption. In addition, this new cement sequesters CO2 when cured into concrete products because CO2 is needed to react with the carbonatable calcium silicate materials during the curing process to form concrete products.
Efflorescence is a surface phenomenon of ordinary portland cement (OPC) concrete that contributes to poor aesthetic and long-term durability of concrete. Typically, efflorescence in an OPC system is a family of crystalline products that are comprised of salts of carbonates, sulfates and chlorides. These crystalline products appear as white deposits that form and grow on the surface of concrete products over time. Efflorescence in OPC systems usually arises from carbonates of calcium, sodium and potassium originating from the cement or concrete components. These deposits typically form as soluble calcium hydroxide migrates from the interior of the sample to the outer surface and subsequently reacts with atmospheric CO2 to form calcium carbonate. White deposits can also be formed in the OPC system from the transport and redisposition of soluble salts such as alkali sulfates or chlorides. These white deposits of soluble salts cause hazing.
Carbonatable calcium silicate-based cements generally do not contain or produce Ca(OH)2 under ordinary circumstances. Any easily carbonatable materials within the cement are reacted and thereby passivated during the CO2 curing process. However, other soluble salts, such as alkali sulfates or chlorides, present in the carbonatable calcium silicate cement or contributed from the concrete mixture components can cause hazing. An effective solution is needed to address this unmet challenge.